Rechargeable lithium-ion (Li-ion) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. As discussed more fully below, a typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode can include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, the electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1794 mAh/g (based on the mass of the lithiated material), for Li2O. Other high-capacity materials include BiF3 (303 mAh/g, lithiated), FeF3 (712 mAh/g, lithiated), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. Nonetheless, the theoretical specific energies are still very high (>800 Wh/kg) compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes, which can enable an electric vehicle to approach a range of 300 miles or more on a single charge.
FIG. 1 depicts a chart 10 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 20 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 20, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 20, lithium/oxygen batteries, even allowing for packaging weight, are capable of providing a specific energy >600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical Li-ion batteries. While lithium/oxygen cells have been demonstrated in controlled laboratory environments, a number of issues remain before full commercial introduction of a lithium/oxygen cell is viable as discussed further below.
A typical lithium/oxygen electrochemical cell 50 is depicted in FIG. 3. The cell 50 includes a negative electrode 52, a positive electrode 54, a porous separator 56, and a current collector 58. The negative electrode 52 is typically metallic lithium. The positive electrode 54 includes electrode particles such as particles 60 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 62. An electrolyte solution 64 containing a salt such as LiPF6 dissolved in an organic solvent such as dimethoxyethane or CH3CN permeates both the porous separator 56 and the positive electrode 54. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 50 to allow a high power.
A portion of the positive electrode 54 is enclosed by a barrier 66. The barrier 66 in FIG. 3 is configured to allow oxygen from an external source 68 to enter the positive electrode 54 while filtering undesired components such as contaminant gases and fluids. The wetting properties of the positive electrode 54 prevent the electrolyte 64 from leaking out of the positive electrode 54. Alternatively, the removal of contaminants from an external source of oxygen, and the retention of cell components such as volatile electrolyte, may be carried out separately from the individual cells. Oxygen from the external source 68 enters the positive electrode 54 through the barrier 66 while the cell 50 discharges and oxygen exits the positive electrode 54 through the barrier 66 as the cell 50 is charged. In operation, as the cell 50 discharges, oxygen and lithium ions are believed to combine to form a discharge product Li2O2 or Li2O in accordance with the following relationship:

The positive electrode 54 in a typical cell 50 is a lightweight, electrically conductive material which has a porosity of greater than 80% to allow the formation and deposition/storage of Li2O2 in the cathode volume. The ability to deposit the Li2O2 directly determines the maximum capacity of the cell. In order to realize a battery system with a specific energy of 600 Wh/kg or greater, a plate with a thickness of 100 μm should have a capacity of about 15 mAh/cm2 or more.
Materials which provide the needed porosity include carbon black, graphite, carbon fibers, carbon nanotubes, and other non-carbon materials. There is evidence that each of these carbon structures undergo an oxidation process during charging of the cell, due at least in part to the harsh environment in the cell (possibly pure oxygen, superoxide and peroxide ions and/or species, formation of solid lithium peroxide on the cathode surface, and electrochemical oxidation potentials of >3V (vs. Li/Li+)).
A number of investigations into the problems associated with lithium/oxygen batteries have been conducted as reported, for example, by Beattie, S., D. Manolescu, and S. Blair, “High-Capacity Lithium-Air Cathodes,” Journal of the Electrochemical Society, 2009. 156: p. A44; Kumar, B., et al., “A Solid-State, Rechargeable, Long Cycle Life Lithium-Air Battery, ” Journal of the Electrochemical Society, 2010. 157: p. A50; Read, J., “Characterization of the lithium/oxygen organic electrolyte battery,” Journal of the Electrochemical Society, 2002. 149: p. A1190; Read, J., et al., “Oxygen transport properties of organic electrolytes and performance of lithium/oxygen battery,” Journal of the Electrochemical Society, 2003. 150: p. A1351; Yang, X. and Y. Xia, “The effect of oxygen pressures on the electrochemical profile of lithium/oxygen battery,” Journal of Solid State Electrochemistry: p. 1-6; and Ogasawara, T., et al., “Rechargeable Li2O2 Electrode for Lithium Batteries,” Journal of the American Chemical Society, 2006. 128(4): p. 1390-1393.
While some issues have been investigated, several challenges remain to be addressed for lithium/oxygen batteries. These challenges include limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air (if the oxygen is obtained from the air), designing a system that achieves favorable specific energy and specific power levels, reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), and improving the number of cycles over which the system can be cycled reversibly.
Some metal/air battery systems, including some lithium/oxygen battery systems, draw air from the surrounding atmosphere to supply oxygen to the battery cells. However, as noted above, one of the challenges of metal/air systems is reducing the influx of contaminants from the air that can lead to degradation of the battery. Some metal/air systems address contamination by supplying pure oxygen to the battery cells solely from an oxygen storage tank, but the addition of the tank adds mass and volume, making it impractical for electric vehicles. Other metal/air systems address contamination by purifying the air inlet with an oxygen separations unit to remove contaminants, such as moisture and carbon dioxide. Some known oxygen separations units are formed by a gas separation nanostructure, polymeric membranes, pressure-swing adsorption units, or temperature-swing adsorption units. These known separations units, however, can be costly, heavy, and large, making it difficult to integrate them into electric vehicles. Moreover, many known oxygen separations units cannot provide the required oxygen flow rates when higher discharge currents are required, such as during acceleration of an electric vehicle.
The following scenarios highlight by example the challenges that existing metal/air systems face in meeting the oxygen demands of a metal/air battery system for an electrical vehicle. The scenarios are based on the following assumptions: (1) The battery chemistry is 2 Li +O2→Li2O2 (on discharge); (2) The battery size is nominally 100 kWh (i.e., 100 kWh of energy can be discharged in a typical duty cycle); (3) The average battery discharge power over the duty cycle is 50 kW; and (4) The peak battery discharge power is 300 kW for up to 1 minute, which represents ˜5% of the battery energy.
In Scenario 1, the lithium/oxygen battery system has an oxygen separations unit, but it does not include an oxygen storage tank. In this case, the separations unit must provide purified oxygen at a rate that supports the peak discharge power of 300 kW, or 750 L/min assuming a discharge voltage of 2.7 V. If an existing ceramic separations unit is used, such as the oxygen generator offered by Solid Cell, Inc. (http://solidcell.com/techOxygen.htm), the system would weigh ˜500 kg and cost ˜$9000, and the volume would be ˜720 L. Even assuming that expected advances in the technology could decrease these figures by a factor of 5, the mass, volume, and cost would still be 100 kg, 144 L, and $1800, respectively.
In Scenario 2, the lithium/oxygen battery system has an oxygen storage tank, but it does not include an oxygen separations unit. In this case, for oxygen stored at 350 bar, the tank mass and volume would be 42 kg and 66 L, respectively. See P. Albertus, T. Lohmann, and J. Christensen, “Overview of Li/O2 Battery Systems, with a Focus on Oxygen handling Requirements and Technologies,” The Lithium Air Battery: Fundamentals, 2014, p. 291-310). Thus, the scenarios illustrate that the weight, size, and/or cost of a lithium/oxygen system using only a gas separations unit (Scenario A) or only an oxygen storage tank (Scenario B) to supply oxygen to the battery cell can be impractical for use in electric vehicles.
What is needed, therefore, is a metal/air battery system capable of providing the requisite amount and purity of oxygen for the battery without exceeding the physical and cost limitations for use in an electric vehicle.